The present invention relates to a glow plug for use with automobile engines and, more particularly, an improvement of a sheath glow plug for adapting it for use with the Wankel type rotary piston engine.
Conventionally, glow plugs are mostly used in diesel engines, particularly those of a type having an auxiliary combustion chamber for the purpose of facilitating engine start-up. In this case, the glow plug is mounted to project into the space of the auxiliary combustion chamber so as to preheat air existing in the auxiliary combustion chamber. As a heating element of the glow plug a durable sheath heater is generally employed which comprises a protective tube and a heater coil mounted in the protective tube. The conventional sheath heater elements employed for diesel engines are mostly of the straight type wherein the protective tube is a straight tube.
In recent developments of Wankel type rotary piston engines, it has been contemplated to employ the glow plug for rotary piston engines instead of the conventional spark plug for the purpose of improving ignition of fuel and thereby improving exhaust gas purification and load performance of the engine. Since the heater element of the glow plug is continuously red-hot due to a continuous supply of electric current, it provides a sustaining ignition source for all combustion chambers of the rotary piston engine over a substantial period in which they traverse the combustion stroke, whereby the occurrence of misfiring is substantially avoided even in idling operation or in low-load low speed operation of the engine.
However, in a rotary piston engine, the glow plug, like a usual spark plug, cannot be mounted to project into the combustion chamber, and it must be mounted in a relatively small plug hole generally formed in the trochoidal peripheral wall of the rotor housing. Therefore, it is only the tip portion of the heater element that is effectively exposed to a flow of fuel-air mixture generated in the combustion chamber during the operation of the engine so as to contribute to igniting the fuel-air mixture. Therefore, when the conventional glow plug having a uniformly heat generating heater element is employed in a rotary piston engine, the tip portion of the heater element is subject to a high rate of cooling by the traversing gas flow when compared with the root portion of the heater element. In this case, there occurs a problem that if a large electric current is supplied in order to maintain the tip portion at a sufficiently high temperature required for effecting good ignition, the root portion of the heater element is overheated and is liable to fuse breaking. Further, if a conventional glow plug such as used in diesel engines and having a straight heater element is employed for a rotary piston engine, there is a problem that only a very small effective heating surface area is available at the tip portion of the heater element.